a. Field of the Invention
The instant disclosure relates to aiming directional acoustic transducers. Among other things, the disclosure relates to aiming directed acoustic transducers in medical applications.
b. Background Art
Medical insertion devices (“MIDS” or “MID” in the singular) such as, catheters, introducers, sheaths, and scopes are inserted into blood vessels, organs, tissue and other body parts for use in various medical applications. For example, MIDS have been used in therapeutic and diagnostic applications such as, forming of tissue lesions (thermally as with radio-frequency “RF”laser or high intensity focused ultrasound (HIFU)); electrically exciting tissue; electrically sensing voltages and/or currents in tissue; implementing surgical procedures; draining and/or delivering fluid or another flowable medium from and/or to the tissue or organ, respectively; delivering another surgical or medical device or tool; delivering sensors or implants into the tissue; delivering balloon-based therapy; delivering a drug or medicament; and/or monitoring important parameters during medical or surgical procedures.
To diagnose and/or provide therapy to the desired tissue (the “target tissue”), the portion of the MID used in diagnosis and/or the provision of therapy usually needs to be positioned at least proximate to the target tissue, if not in direct contact with the target tissue. With catheters, this tissue contacting or proximate portion which would contain the inventive transducer assembly is generally referred to as the distal end or tip of the catheter. In addition, the MID may need to obtain certain diagnostic or measurement information about the target tissue before, during, and/or after therapy is provided. A variety of ultrasonic transducers have been employed in the pulse-echo or pinging mode for providing the diagnostic or measurement capability from a catheter tip. The emitted and received acoustical waves of such transducers are either directed or nondirected (omnidirectional) at angles to the long catheter axis or the more-rigid tip axis. Directed transducers emit/receive along one or more angular directions relative to a line typically perpendicular or at a small angle to the catheter tip axis. Directed transducers include fixed (relative to catheter) mechanically focused transducers, mechanically steerable rotating (relative to catheter) mechanically focused transducers, and fixed (relative to catheter) electronically steerable phased array 2D and 3D imaging transducers. The directions at given times are known, and thus the acoustic reflection data received is known to be that along one or more specific (scan) lines or directions penetrating the adjacent tissue. Directed transducers may provide image information along each such directed (scan) line or direction.
Nondirected transducers are typically omnidirectional, meaning they simultaneously (as opposed to serially) emit/receive at all angles throughout all or most of 360 degrees of arc about the catheter tip axis, all such angles typically being orthogonal or nearly orthogonal to the catheter tip axis. Typically the transducer is cylindrical with its cylindrical axis arranged along the catheter tip axis, and it emits/receives from its 360 degree cylindrical external surface substantially generally orthogonal to the common cylindrical transducer and tip axis. Because the acoustic reflection data received by an omnidirectional transducer comes from all angular or rotational directions simultaneously and is inevitably electronically summed before analysis or presentation, it is unknown which reflections or how much of a given reflection at a certain radial depth (an echo time-delay amount) comes from a particular angular direction. Nondirected or omnidirectional transducers do not provide an image, partial image, or image information. However, construction is relatively simple and the total reflected power from each radial distance summed from all angles may be known.
Directed and omnidirectional transducers emit acoustic or ultrasonic pulses along one or more directions and then listen for resulting echoes or reflections coming from various tissue depths from those one or more directions. Such echoes or reflections come from internal tissue interfaces, blood vessels, natural lesions and tumors, and from ablated lesions having ablation-induced microbubbles or acoustic impedance differences. Since the acoustic velocity in natural tissue is known (i.e., about 1540 meters/sec with very little variation), the distance or depth to a tissue reflector can be determined knowing its echo delay time and dividing that time by a factor of two since the acoustic waves undergo a two-way round trip. The thickness of a tissue layer or blood chamber may be measured by looking at the difference between the depths or distances of its frontside and backside reflections or echoes.
A reflector depth or distance along a single direction for an omnidirectional transducer cannot be determined because the omnidirectional transducer is essentially simultaneously measuring all directions at once and adding the received signals. The signal-to-noise (S/N) of directed transducers is superior to an omnidirectional transducer both because directed transducers are looking only at the target and because all the energy that would have been involved with a 360 degree omnidirectional transducer is instead restricted to a small angle of the directed beam such that all the energy is injected and received along the specific direction(s) of interest (i.e., the energy efficiency is better).
Directed and omnidirectional transducers are typically operated in the frequency range of 2 MHz to 20 Mhz. The higher end of these frequencies provides less penetration but more axial depth resolution, so the higher frequencies are used for the highest possible accuracy in thin or shallow tissues. A piezotransducer transmit pulse, typically of tens of volts in amplitude, will contain one or a few sinusoidal waveforms at the above center frequency. The received echoes typically include those from a number of buried interfaces at various depths plus one or more reverberations (e.g., false reflections) coming from the transducer itself as is well known to the art. Emitting pulse lengths and echo delay times are typically measured in microseconds or less for 1 MHz and above ultrasound as is known in the echo arts.